Reduction of high soil test phosphorus by bermudagrass and ryegrass–bermudagrass following the cessation of broiler litter applications

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Factors that contribute to high soil P in bermudagrass [Cynodon dactylon L. (Pers.)] pastures include continued use of broiler litter rich in P to meet forage N requirements and removal of hay in summer only. This study determined if harvesting annual ryegrass (Lolium multiflorum Lam.) in addition to bermudagrass would reduce surplus soil P, Cu, and Zn faster than bermudagrass alone after litter application ceased on a Ruston soil (fine-loamy, siliceaous, semiactive, thermic Typic Paleudults). During a 3-yr build-up, ‘Coastal’ bermudagrass was fertilized with 0, 4.5, 9, 18, and 36 Mg ha–1 litter yr–1. During the drawdown phase, plots were split and half was overseeded with ryegrass in fall 2001, 2002, and 2003. Whole plots were fertilized with NH4NO3 in spring–summer to provide 268 kg ha–1 N yr–1. Forage yield and P uptake increased as antecedent litter rate increased, and were greater in ryegrass–bermudagrass than bermudagrass in 2002, but not 2003. At 9 Mg ha–1 litter, harvesting ryegrass in addition to bermudagrass increased P uptake by 10 to 55%, depending on study year. During the drawdown phase, soil Mehlich-3 P (M3P) and water-extractable P to 15-cm depth decreased by as much as 50 and 70%, respectively. Soils analysis within each sampling date found no significant effect of forage system or its interaction with litter rate. Data for 9 Mg ha–1 litter rate indicated 2 yr of forage P removal decreased residual M3P to an acceptable agronomic level (<70 mg kg–1). The potential to decrease surplus soil P by ryegrass–bermudagrass hay harvests was greatest when rainfall was inadequate for optimum bermudagrass yield.

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